Make the batter by beating the egg into the milk with a little salt. Add the mustard powder to the flour and then make a well in the centre of the flour and beat in the milk/egg mixture. Season and set aside in the fridge until ready to use.

Remove the sausages from their skins and roll each into 2 little balls

Heat 2 tbsp of the oil in a frying pan and brown the sausage balls until golden. Set aside.

Next soften the leeks in the same pan using the oil from the sausages and add a little knob of butter once soft and slightly golden. Set aside with the sausages.

Put the remaining olive oil into an ovenproof dish and heat in the oven for a good few minutes until piping hot. Add the leeks and sausage balls and then pour over the batter. Cook for about 30-40 minutes until golden brown and puffy.